1. Technical Field
The invention relates to a technology of a liquid crystal device with photodetection function, by which the position of a pointing object, such as finger, is located or the region of the pointing object is detected, and also to a technology of an electronic apparatus provided with the liquid crystal device.
2. Related Art
In liquid crystal devices that are widely used as a display device of an electronic apparatus, or the like, a liquid crystal device having a so-called touch panel function has been suggested. In the touch panel function, a photosensor is arranged for each plurality of pixel portions or for each group consisting of a selected number of pixel portions, and thereby it is possible to display an image using light that is transmitted through the pixel portions and to enter information into the liquid crystal device through the pointing object, such as finger. In the above liquid crystal device, the photosensor detects that the pointing object, such as finger or pointing member, contacts the display surface of the liquid crystal device or moves on the display surface. Thus, information may be entered into the liquid crystal device. For example, Touch Panel Function Integrated LCD Using LTPS Technology, N. Nakamura et al, IDW/AD'05 p. 1003-1006 describes a liquid crystal device that is able to display an image through the operation of a driving circuit formed of TFTs each having a low temperature polysilicon (LTPS), and that has a touch panel function by which various pieces of information may be entered on the basis of the image of a pointing object, which is acquired by a photosensor arranged in each pixel.
The photosensor, which is mounted on the above liquid crystal device, is, for example, formed to include a circuitry such that a photodiode and a capacitor are electrically connected to each other. Electric charge stored in the capacitor is discharged depending on a photoelectric current generated in the photodiode that has received an incident light, and the gray-scale level of an image is determined on the basis of an electric potential that has changed because of the discharge. More specifically, for example, the photosensors that are arranged in a region that overlaps the pointing object within the display area in which an image is displayed, that is, the photosensors that are arranged in a region that overlaps the shadow of the pointing object detect the amount of incident light at the shadow of the pointing object, and the photosensors that are arranged in a region that does not overlap the pointing object detect the amount of outside light, which is not blocked by the pointing object, as incident light. Thus, an image having a difference in gray-scale level among pixel portions, depending on a difference in the amount of light, is acquired. Thus, in the liquid crystal device of this type, the amount of incident light that enters the display surface on which an image is displayed is detected, and then the position of the pointing object may be located on the basis of the image consisting of image portions of which the gray-scale levels are determined depending on the amounts of incident light detected by the photosensors.
The detectable range in which the amount of light that may be detected by the photosensors mounted on the liquid crystal device of this type, that is, the range in which the amount of incident light that can generate a photoelectric current corresponding to the amount of incident light is defined through the design of the photosensor. Thus, when the photosensor receives incident light that has an amount of light higher than the detectable range, a photoelectric current generated depending on the amount of light becomes saturated and therefore a variation in voltage generated on the basis of the photoelectric current does not occur. Hence, the image portion of the pointing object cannot be distinguished from another image portion.
In addition, in the display area of the liquid crystal device, when another portion that differs from a pointing object overlaps the pointing object, the shadow of the pointing object and the shadow of the another portion cannot be distinguished from each other.
When an image is formed of only one of a white image portion (bright image with a high gray-scale level) and a black image portion (dark image with a low gray-scale level) on the basis of the amount of incident light received by the photosensors, it is conceivable that the amounts of incident light that enters a plurality of photosensors formed in the display area are adjusted to be uniform to thereby adjust the amounts of incident light detected by the photosensors so as to fall within the detectable range, and, in this way, the gray-scale levels of the image are adjusted so that the image portion of the pointing object may be distinguished from another image portion.
However, in the liquid crystal device of this type, when an image that includes both a white image portion and a black image portion is acquired because of surrounding environment of the pointing object, more specifically, because of the intensity of outside light or the presence of another portion (that is, noise) that overlaps the pointing object, it becomes difficult to determine in which the image portion of the pointing object is included, the white image portion or the black image portion. Thus, a technical problem arises in which it becomes difficult to locate the position of the pointing object by distinguishing the image portion of the pointing object from another image portion.
Particularly, when the image data of an image that includes the image portion of the pointing object have gray-scale levels to a degree such that the image portion of the pointing object may be distinguished, the image portion of the pointing object may be distinguished from another portion by performing various computations on the image data; however, when the amount of light that falls outside the range within which the amount of light can be detected by the photosensor, even the image data that contain the gray-scale level data by which the image portion of the pointing object may be located through the computations cannot be acquired.
Here, it is conceivable that, as in the case of an imaging apparatus, such as a camera, that has a mechanical aperture mechanism and a mechanical shutter mechanism provided midway of the optical system, each photosensor portion is provided with an aperture mechanism and a shutter mechanism. However, it is difficult to reserve a space for providing the aperture mechanism, and the like, at the light receiving side of the photosensor portion along the optical path of incident light. Particularly, in the liquid crystal device of this type, the photosensor portions need to be provided in the display area of the liquid crystal device, so that it is difficult to reserve a space for providing the aperture mechanism, and the like, without deteriorating the display performance of the liquid crystal device, more specifically, without greatly reducing the aperture area through which display light that substantially contribute to displaying an image is transmitted in the display area.
Furthermore, even in an image sensor that detects the image of a detected object, as well as the liquid crystal device that has a touch panel function, there is a technical problem that it becomes difficult to detect the image portion of the detected object by distinguishing the image portion from another image portion.